The present invention relates to a suspension bridge, and more particularly, to the structure of a suspension bridge of which static characteristics and aerodynamic stability are improved by applying a temporary dead load as an additional mass when the bridge is exposed to conditions such as violent storms that would cause particularly rigorous swaying of the suspension bridge.
As a countermeasure against strong winds, suspension bridges are provided with an additional mass such as water and concrete in the stiffening girder to control the vertical and torsional vibrations of the girder. Such suspension bridges are known from, for example, Japanese Patent Publication Sho 47-44944, Japanese Patent Application Laid-open Sho 60-192007, U.S. Pat. No. 4,665,578, and Japanese Patent Application Laid-open Sho 63-134701.
Suspension bridges disclosed in JP Publication Sho 47-44944 and JPA Laid-open Sho 63-134701 utilize the dynamic energy of water pooled in advance in the stiffening girder to absorb the vertical and torsional vibrations of the girder during a storm, while those according to JPA Laid-open Sho 60-192007 and U.S. Pat. No. 4,665,578 reduce such vertical and torsional vibrations by arranging a pre-fixed amount of additional mass in the girder.
These bridge structures all utilize an additional mass such as water and concrete placed in the stiffening girder or the tower columns to reduce the vertical and torsional vibrations in the girder. As such, the additional mass is included as a part of the design dead load.
Generally, bridges are designed by considering the normal conditions when the dead load and the live load mainly of moving vehicles are working, and the stormy conditions when the wind load as well as the dead load play a vital role. The smaller the dead load of the main cable, anchors, towers, hangers, etc. that are designed by considering the vertical load, the better it is in terms of economy under the normal conditions. Conversely, the heavier the dead load, the static characteristics and aerodynamic stability against vibrations improve under stormy conditions. In the case of a stiffening girder of suspension bridge which is mainly designed to safeguard against stormy conditions, the girder can be made smaller in sectional area if a heavier temporary dead load is assigned, which in turn contributes to cost reduction of the girder itself.
Conventional countermeasures of applying an additional mass of water, concrete or the like to the stiffening girder in advance as the dead load are defective in that economical advantages of the main cable, anchors, towers and hangers that are designed based on the vertical loads under the normal conditions are sacrificed because of the increased dead load.